In recent years, with the increasingly higher integration and higher power of electronic devices and semiconductor devices, heat dissipation technology for dissipating heat generated from semiconductor elements are becoming extremely important. As such, there is a demand for a heat dissipation substrate with excellent heat dissipation properties that can be used as an insulating member in a semiconductor device, for example.
For example, metal or ceramics may be contemplated as the material of such a heat dissipation substrate. However, metal may be inferior to ceramics in terms of oxidation resistance, water resistance, and corrosion resistance, and more particularly, metal cannot be used without cooling under conditions exceeding 500° C. Further, because metal is electrically conductive, it is not suitable for use in an insulating substrate requiring high heat dissipation properties such as a high-density mounting substrate requiring insulation, for example.
On the other hand, ceramics have higher oxidation resistance, water resistance, and corrosion resistance as compared with metal. As such, for example, alumina and aluminum nitride have been used as materials of heat dissipation substrates. In particular, aluminum nitride has both excellent insulation and high thermal conductivity, and is therefore used as a heat dissipation substrate material for power modules. However, aluminum nitride has inferior mechanical properties, such as strength and fracture toughness, and is not sufficiently reliable such that its use is very limited.
On the other hand, a silicon nitride sintered bodies are widely known as excellent structural ceramic materials having both high strength and fracture toughness. Single crystals of such materials have a very high thermal conductivity estimated to be from 200 to 320 W/mK. As such, silicon nitride sintered bodies are expected to be used as materials of heat dissipation substrates. However, in a typical silicon nitride sintered body, impurities such as oxygen are dissolved in the silicon nitride particles. Thus, phonons for inducing thermal conduction may be scattered, and the terminal conductivity of the silicon nitride sintered body may be reduced to 20 to 80 W/mK, which is much lower than the estimated thermal conductivity of single crystals.
Non-Patent Literature Document 1 discloses that in order to achieve high thermal conductivity in a silicon nitride sintered body, a glassy phase of low thermal conductivity has to be reduced and the amount of oxygen dissolved in silicon nitride particles have to be reduced during sintering.
Note that silicon nitride has very high covalency and cannot be easily sintered. Thus, in order to obtain a dense sintered body, liquid-phase sintering using a sintering aid has to be performed.
Oxides may be used as the sintering aid to be added in producing a sintered body of silicon nitride. The added sintering aid forms a liquid phase by reacting with the silica on the surface silicon nitride powder during sintering, and this liquid phase promotes densification and grain growth. A large part of the liquid phase formed during sintering remains in the sintered body as a glassy phase upon cooling.
Further, in producing a sintered body of silicon nitride, rare earth oxides, which have high oxygen affinity, may be used as the sintering aid to be added in order to achieve higher thermal conductivity in the silicon nitride sintered body. When a rare earth oxide is added, the liquid phase that is formed traps a large amount of oxygen, and in this way, the amount oxygen dissolved in the silicon nitride particles may be reduced and higher thermal conductivity may be achieved in the silicon nitride sintered body.
However, when only a rare earth oxide is added as the sintering aid, the liquid phase that is formed has a high melting point such that it is difficult to obtain a dense sintered body having excellent mechanical properties. Thus, various approaches have been implemented for obtaining a silicon nitride sintered body with both excellent mechanical properties and high thermal conductivity.
For example, Patent Document 1 discloses a method for producing a high thermal conductivity silicon nitride sintered body that includes forming a compact by adding an oxide sintering aid to silicon powder containing Al at 0.1 wt % or less, the oxide sintering aid being made of at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er and Yb at 0.5-10 wt %, and then sintering the compact under a nitrogen gas pressure ranging from 1 atm to 500 atm and a temperature ranging from 1700° C. to 2300° C. until a porosity of 5% or less and a predetermined structure is obtained. In this way, a high thermal conductivity silicon nitride sintered body with a thermal conductivity of at least 80 W/mK, a fracture toughness of at least 7 MPam1/2, a bending strength (measured using the 4-point bending technique) of at least 600 MPa can be obtained.
Also, Patent Document 2 discloses a method for producing a high thermal conductivity silicon nitride sintered body by forming a compact of a raw material powder including silicon nitride powder by adding an oxide of at least one element selected from the group consisting of magnesium, yttrium, and lanthanoid elements at a total weight percent of 1.0 wt % or less, and then sintering the compact at a temperature of 1800° C. to 2000° C. and at a nitrogen gas pressure of 0.5 MPa to 10 MPa using a mixed powder consisting of silicon nitride, boron, and magnesium oxide as packing powder for adjusting the sintering atmosphere. Further, an example is disclosed where a silicon nitride sintered body with a thermal conductivity of at least 90 W/mk at room temperature and a 3-point bending strength of at least 600 MPa is obtained by controlling the particle size of the sintered body, the amount of oxygen in the silicon nitride particles, and the composition of the residual sintering aid to be within predetermined ranges.
Also, Non-Patent Literature Document 2 discloses that in the conventional sintering method, although grain growth of silicon nitride is promoted and thermal conductivity is improved as the sintering time is increased, the grain growth may be excessive such that the strength and fracture toughness of the silicon nitride sintered body substantially decreases in conjunction with the improvement of the thermal conductivity.
As exemplified by the above case, in the conventional method, a silicon nitride sintered body with a thermal conductivity of at least 80 W/mK and a bending strength of at least 600 MPa has been produced using silicon nitride powder, and optimizing process parameters, such as the type of sintering aid, the amount of sintering aid added, and sintering conditions.
However, in the methods disclosed in Patent Documents 1 and 2, manufacturing costs are increased because expensive silicon nitride powder is used. Also, when sintering conditions are adjusted to achieve a thermal conductivity higher than 100 W/mK, the strength and fracture toughness drastically decrease to thereby compromise mechanical reliability.
As can be appreciated, high thermal conductivity silicon nitride bodies obtained by conventional sintering methods do not adequately meet the two demands of manufacturing cost reduction and concurrence of high thermal conductivity and mechanical properties.
Thus, in terms of reducing the cost of the raw material powder, a high thermal conductivity silicon nitride material is being developed that uses inexpensive silicon powder as the raw material powder and uses a so-called reaction sintering process that involves nitriding the molded material in nitrogen and sintering the molded material at a high temperature thereafter.
For example, Patent Document 3 discloses a method for producing a Si3N4 sintered body by mixing 80 wt % to 99 wt % of Si powder containing oxygen at 1 wt % or less with 1 wt % to 20 wt % of oxide powder made of an oxide of at least one element selected from the group consisting of Y, Yb, and Sm, performing a nitriding process on the molded material at a temperature of 1400° C. or lower in a nitrogen atmosphere, and then sintering the resulting nitride material at a temperature of 1700° C. to 1950° C. in a nitrogen atmosphere. In such a method for producing a Si3N4 sintered body, a high purity Si powder with an oxygen content of 1 wt % or lower is used in order to suppress the dissolution of oxygen ion impurities into the Si3N4 crystal grains and increase the thermal conductivity of the Si3N4 sintered body. Also, it is disclosed that by additionally mixing into the raw material powder a reducing coating agent at 1 wt % to 10 wt % of the Si powder, and heating the compact at a temperature of 200° C. to 800° C. in a vacuum of 100 Torr or less or in a nitrogen atmosphere before performing the nitriding and sintering processes as described above, the oxygen content of the resulting Si3N4 sintered body can be further reduced and the thermal conductivity can be further improved.
Also, Patent Documents 4 and 5 disclose producing a silicon nitride sintered body by mixing an oxide of a rare earth element and a magnesium compound into silicon powder or a mixed powder of silicon powder and silicon nitride powder, the oxide of the rare earth element being mixed at 0.5 mol % to 7 mol % and the magnesium compound being mixed at 1 mol % to 7 mol % when the amount of silicon is converted into an amount of silicon nitride. The silicon nitride sintered body is produced by forming and nitriding the mixed material, and heating the resulting nitride body in a nitrogen atmosphere at a predetermined pressure to densify the nitride body such that a relative density of 95% or higher is achieved. In this way, a silicon nitride sintered body with a thermal conductivity of at least 100 W/mK, a 3-point bending strength of at least 600 MPa, and a fracture toughness of at least 7 MPam1/2 can be obtained.
However, when a silicon nitride substrate is produced according to the methods for producing silicon nitride sintered bodies as described in Patent Documents 3-5, a porous modified layer having a thickness of about 90 μm to 140 μm and containing numerous pores is formed on a surface layer portion of the silicon nitride substrate. When a modified layer is included in the surface of the silicon nitride substrate, the electrical characteristics and mechanical strength of the silicon nitride substrate will be degraded, and as such, the portion of the silicon nitride substrate surface where the modified layer is formed has to be scraped off through polishing or the like. This in turn leads to an increase in the number of process steps and an increase in costs.